Power add-on device for manual wheelchair

ABSTRACT

A power add-on device for powering a manual wheelchair includes a motorized component including dual electric motors and a power source electrically coupled to the electric motors, wherein each of the motors is configured to turn a respective one of a set of drive wheels. The power add-on device includes a latching mechanism adapted to attach the power add-on device to the camber tube of the manual wheelchair; and a controller, reachable by a person sitting in the manual wheelchair, that controls the latching mechanism, the motors, and a swing arm that allows the rear wheels of the manual wheelchair to be lifted off the ground. The front wheels of the manual wheelchair are allowed to be lifted several inches off the ground to avoid obstacles, or can be lifted by the user leaning back. Advantageously, the power add-on device can be detached from the manual wheelchair and loaded into the trunk of a car when travelling or may be checked in as baggage when flying.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/788,147, filed on May 26, 2010 (entitled “Power Add-OnDevice For Manual Wheelchair”), which claims the benefit under 35 U.S.C.§119 of U.S. Provisional Patent Application No. 61/181,602 filed byPatrick Tallino on May 27, 2009 and entitled “Power Add-On Device ForManual Wheelchair”, the entirety of each of which is hereby incorporatedby reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an auxiliary power add-on attachmentfor a manual rigid-framed wheelchair.

2. Description of the Related Art

Most spinal cord-injured individuals with injury levels from the fivecervical vertebrate down to the lumbar vertebrates use a lightweightrigid framed manual wheelchair for everyday use. However, lengthyoutings or ones that require traversing uneven or sloped terrain areunrealistic and often impossible for many using manual chairs. Althoughelectric-powered wheelchairs exist that can be used for thosesituations, they are not generally prescribed unless the user lacks theability to use a manual wheelchair. Moreover, electric-poweredwheelchairs tend to be expensive, heavy, and cumbersome.

Several patents disclose devices which can be attached to a manualwheelchair to electrically power the chair. For example, U.S. Pat. No.5,494,126 to Meeker, entitled “Apparatus and Method For Attaching aMotorized Wheel to a Wheelchair”, discloses a motorized wheel that canbe attached to the front of a wheelchair. As another example, U.S. Pat.No. 5,496,904 to Zwaan, entitled “Wheelchair Power System”, discloses apower system that can be added to a manual wheelchair to convert it toan electric-powered wheelchair.

Although such devices are somewhat useful and beneficial, the existingtechnology fails to provide an easy way to attach/detach a power add-ondevice to a conventional manual wheelchair. Furthermore, such devicesfail to provide for traversal over rough terrain and adequate stability.Additionally, many such devices require modification to the manualwheelchair. Accordingly, it would be desirable and highly advantageousfor there to be an auxiliary power add-on attachment for a manualwheelchair that overcomes these and other deficiencies.

SUMMARY OF THE INVENTION

In an embodiment of the present invention, a power add-on device forpowering a manual wheelchair includes a motorized component includingdual electric motors and a power source electrically coupled to theelectric motors, wherein each of the motors is configured to turn arespective one of a set of drive wheels. The power add-on deviceincludes a latching mechanism adapted to attach the power add-on deviceto the camber tube of the manual wheelchair; and a controller, reachableby a person sitting in the manual wheelchair, that controls the latchingmechanism, the motors, and a swing arm that allows the rear wheels ofthe manual wheelchair to be lifted off the ground.

Attachment of the wheelchair to the power add-on device is accomplishedsimply by backing up the wheelchair into the device. To reduce tipping,the power add-on device features a front anti-tip castor wheel and aback anti-tip castor wheel. When the power add-on device is attached tothe wheelchair, the front wheels can be lifted several inches off theground when encountering obstacles or by the user leaning back. Anotable design feature of the present invention is that the latchingmechanism is not tightly clamped down on the camber tube; instead, itrather surrounds the camber tube, allowing the camber tube to rotateslightly as the wheelchair tilts to allow the front wheels of the manualwheelchair to be lifted. Advantageously, the power add-on device can bedetached from the manual wheelchair and loaded into the trunk of a carwhen travelling or may be checked in as baggage when flying.

In an embodiment of the present invention, the latching mechanismincludes a clamshell latching mechanism. In this embodiment, theclamshell latching mechanism includes an upper clamshell portion and abottom clamshell portion, the upper clamshell portion and the bottomclamshell portion attached by a hinge. When the clamshell latchingmechanism is in a closed position, the clamshell latching mechanismsurrounds the camber tube of the manual wheelchair.

These and other aspects, features, and advantages of the presentinvention will become apparent from the following detailed descriptionof preferred embodiments, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary power add-on device for a manual wheelchair,in accordance with a preferred embodiment of the present invention;

FIG. 2A shows a conventional manual wheelchair having a camber tube;

FIG. 2B shows the power add-on device of FIG. 1 attached to aconventional manual wheelchair;

FIG. 3 shows the frame structure of the power add-on device of FIG. 1;

FIGS. 4 to 6 show an exemplary power add-on device for a manualwheelchair, in accordance with another preferred embodiment of thepresent invention;

FIGS. 7 and 8 show the frame structure of the power add-on device ofFIG. 4; and

FIG. 9 shows a schematic view of the electrical components for the poweradd-on device.

FIG. 10 shows an embodiment of a power add-on device having anembodiment of a rotation resistance element.

FIG. 11 shows another view of the embodiment of the power add-on deviceshown in FIG. 10.

FIG. 12 shows an embodiment of a bracket assembly and roller member ofthe rotation resistance element of the embodiment of the power add-ondevice shown in FIG. 10.

FIG. 13 shows the embodiment of the rotation resistance element of theembodiment of the power add-on device shown in FIG. 10, illustrating anembodiment of a gas spring component secured to the bracket assembly ofthe rotation resistance element.

FIG. 14 shows another view of the embodiment of the rotation resistanceelement of FIG. 13.

FIG. 15 shows another view of the embodiment of the rotation resistanceelement shown in FIG. 13.

FIG. 16 shows an enlarged view of the embodiment of the power add-ondevice shown in FIG. 10, showing the embodiment of the rotationresistance element of FIG. 13.

FIG. 17 shows a portion of the bracket assembly of the rotationresistance element attached to the power add-on device.

FIG. 18 shows a portion of the bracket assembly of the rotationresistance element attached to the power add-on device.

FIG. 19 shows a rear view of a portion of the rotation resistanceelement in a first or relaxed position relative to a wheelchair.

FIG. 20 shows a side view of a portion of the rotation resistanceelement in a first or relaxed position relative to a wheelchair.

FIG. 21 shows a side view of a portion of the rotation resistanceelement in a second or rotated position relative to a wheelchair.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates an exemplary power add-on device 100 useable forpowering a manual wheelchair, in accordance with a preferred embodimentof the present invention. FIG. 2B illustrates the power add-on device100 attached to a manual wheelchair 201 having a camber tube 202. Thepower add-on device 100 includes a housing 102. The housing 102includes, therein, two conventional electric wheel-chair motors that areelectrically coupled to a power source, and are arranged so that eachdrives a wheel 104. The manual wheelchair 201 can be attached to thepower add-on device 100 simply by backing up the manual wheelchair 201until a pair of latches 103 holds the camber tube of the manualwheelchair 201. Advantageously, no modification is required to themanual wheelchair 201.

Preferably, the latches 103 can be activated by an electric actuator sothat when a user wants to de-couple the power add-on device 100 from themanual wheelchair 250, the user employs remote control 108 to activatethe electric actuator so that the latches 103 assume an open position,releasing the manual wheelchair 250. Preferably, the remote control 108is a conventional joystick or other such user-friendly remote controldevice.

Preferably, the power add-on device 100 can also be released manually,for example, by pulling a lever.

Preferably, the power add-on device 100 has built in recline andanti-tip features so the user can recline and relieve pressure fromtheir seat cushion safely which is very important in order to avoidpressure sores. Preferably, the anti-tip feature is at least in partaccomplished by employing front anti-tip castor wheel 107 and rearanti-tip castor wheel 106.

When the power add-on device 100 is attached to the manual wheelchair250, the front anti-tip castor wheel 107 can be lifted several inchesoff the ground when encountering obstacles (preferably, as much as threeinches). A notable design feature of the present invention is that thelatches 103 are not tightly clamped down on the camber tube; instead,the latches 103 rather surround the camber tube, allowing the cambertube to rotate slightly as the manual wheelchair 250 tilts to allow thefront wheels of the manual wheelchair to lift of the ground whenencountering obstacles or when the user wishes to recline.

Preferably, the power add-on device 100 includes built-in armrests 109on which the remote control 108 is mounted which operates the device.Preferably, the armrests 109 are mounted to the housing 102 usingbrackets 111, as shown.

Referring to FIG. 3, the framing structure 400 of the power add-ondevice 100 is shown. As depicted in FIG. 4, the framing structure 400includes body frame 401, motor mount 402 (for securely holding theelectric motors), coil-over shock absorber 403, rear anti-tip castorwheel 106 and front anti-tip castor wheel 107 (to prevent tipping, asdiscussed above), combination electric actuator 404 and latches 103 (toopen the latching system and release the manual wheelchair 201, asdiscussed above), armrest mounts 111 (to secure the pair of armrests109), and combination electric actuator 406/swing arm 408 (to allow theswing arm 408 to pivot upwardly, thereby raising the rear wheels of themanual wheelchair 250 off the ground and transferring the weight of theuser from the rear wheels of the manual wheelchair through theshock-absorbed swing arm to provide traction to the power add-on's drivewheels). However, even when the rear wheels of the manual wheelchair 250are positioned on the ground, relatively level terrain may still betraversed easily because the powered drive wheels 104 are lined up withthe wheels of the manual wheelchair 250. In general, the higher the rearwheels of the manual wheelchair 250 are raised, the larger the obstaclesthat can be traversed. An additional benefit of this design is that itenables the user the advantage of additional height and reachcapabilities for different everyday tasks.

FIG. 4 Shows an alternate and preferred latching mechanism wherein theuser backs up to an upper clamshell 201 of the latch 203 and thenemploys the remote control 108 to activate an electric actuator to closethe latch 204 by lifting a lower clamshell 204, thereby securing thecamber tube of the manual wheelchair to the power add-on device 100. Asillustrated, the upper clamshell 201 and a bottom clamshell 204 areattached by at least one hinge. As shown in FIG. 4, the latch 203 is inan open position. FIG. 5 illustrates the latch 203 in a closed position.FIG. 6 illustrates that the front castor wheel 107 can be lifted off theground by this mechanism as well to provide extra clearance ofobstacles.

FIG. 7 illustrates the framing structure 400 of the power add-on device100 with the alternate preferred latching mechanism discussed above. Asdepicted in FIG. 7, the electric actuator 404 is coupled to a pivotmember 405, and the pivot member 405 is coupled to the coil-over shockabsorber 403. In operation, as shown in FIG. 8, when the electricactuator 404 is activated, the electric actuator 404 applies force tothe pivot member 405, and the pivot member 405 changes the direction ofthe force to upwardly apply the force to the coil-over shock absorber403. As shown, the coil-over shock absorber 403 is coupled to the bottomclamshell 204, and the coil-over shock absorber 403 pushes the bottomclamshell 204 so as to close the latch 204.

FIG. 9 shows a schematic view of the electrical components for the poweradd-on device 100. As depicted in FIG. 5, the electrical system includespower source 501, motors 502, brakes 503, linear actuators 504, 505,controller 506, and charger 507.

The power source 501 comprises energy storage via batteries withcharging and current limiting elements. The batteries are electricallyconnected in series, as shown, and provide all power for all functions.This configuration of battery power allows for use of standard batterieswhile providing the total output voltage needed for proper operation ofthe motors 502, actuators 503, 504, and brakes 503.

Since it is possible that either the motors 502 or the actuators 503,504 can experience states of operation, such as short circuiting duringa failure mode, which draw excessive power from the batteries, thebatteries are each protected with current limiting elements. Thesecurrent limiting elements comprise left battery fuse 515, right batteryfuse 516, and circuit breaker 518. The circuit-opening characteristicsof these current limiting elements preferably are selected based onallowing the circuit breaker first open-circuit followed by the fusesopen-circuiting as the total current sourced from the batteries exceedthe rated current discharge rate of the batteries.

The motors 502 are preferably direct current motors, sized preferablyfor propelling the manual wheelchair 201 and an adult user up at least atwenty degree grade. Similarly, the brakes are preferablydirect-current-activated at the voltage of the two batteries whenconnected in series.

The actuator 504 is preferably a direct-current-powered actuator sizedand preferably mounted under the frame so as to raise the swing arm 408,and thus lifting the rear of the manual wheelchair 201.

The actuator 505 is preferably a direct-current-powered actuator sizedand preferably mounted so as to operate the latches 103, therebyreleasing the wheelchair 201.

The controller 506 preferably includes wired or wireless remote actuatorswitches attached to a joystick/controller. Alternately, preferably, theactuator switches can be built into the joystick/controller. In eitherpreferred configuration the actuator switches allow the operator tocontrol the raising of the swing arm 408 (thus lifting the rear of themanual wheelchair 201), and operating the release lever (opening thelatches 103 and releasing the manual chair 201), as shown.

Some embodiments of the power add-on device can be configured tominimize, dampen, and/or slow the rotation of the power add-on devicerelative to a manual chair coupled with the power add-on device. Forexample, with reference to FIGS. 10 and 11, in some embodiments, thepower add-on device 600 can be configured to have a rotation limiting ordampening element or mechanism 602 (also referred to herein as ananti-tilt mechanism or a rotation resistance element) configured tominimize, dampen, and/or slow rotation between the power add-on device600 and a manual wheelchair coupled with the power add-on device. Notethat the manual wheelchair is not illustrated in either of FIG. 10 or11.

As discussed above, at least some of the power add-on device embodimentsdisclosed herein can be coupled with a camber tube or similar componentof the wheelchair, for example, by engaging the camber tube of thewheelchair with the engagement mechanism (which can be a clamshellengagement mechanism, latch mechanism, or other similar or suitablemechanism). Engaging the camber tube of the wheelchair can permit adesirable amount of rotation of the manual chair relative to the poweradd-on device. Such relative rotation can be beneficial, for example,for traversing changes in the slope or height of the ground surface. Forexample, when the power add-on device and wheelchair advance up or downa ramp, whether an upward sloping ramp or downward sloping ramp, therewill likely be a point when the drive wheels of the power add-on deviceare at a different height or level than the front wheels of thewheelchair. Without permitting the rotation of the wheelchair relativeto the power add-on device, such mismatch in the height of the drivewheels of the power add-on device relative to the front and/or the rearwheels of wheelchair can result in decreased stability and control ofthe wheelchair, and increased stress on the connection between the poweradd-on device and the wheelchair. Permitting such relative rotationtherefore increases the stability, comfort, and overall performance ofthe power add-on device and wheelchair (i.e., the powered wheelchairsystem) and the ability of the powered wheelchair system to traverseuneven ground surfaces, bumps, and other changes in the ground surface.

However, there are also advantages to softening or resisting the amountof rotation between the power add-on device and the wheelchair, as toomuch relative rotation or quick rotation between the power add-on deviceand the wheelchair can affect the comfort level of the user, the controlof the power add-on device, and the control of the remote control, suchas the remote control 108. For example, in embodiments where thearmrests 109 are secured to the power add-on device, and where theremote control is secured to the armrests, such as armrests 109, rapidor sudden, undampened rotation between the power add-on device and thewheelchair can cause relative rotation and movement between a userseated in the wheelchair and the armrests and remote control. This canreduce the comfort level of the user and the level of control the userhas over the remote control.

In some embodiments, softening or dampening the amount of rotationbetween the power add-on device and the wheelchair can be achieved withthe rotation resisting element 602 that can engage a rear facing surfaceof the wheelchair and prevent the power add-on device from over-tippingor over-tilting in a forward direction relative to the wheelchair and/orprevent the wheelchair from over-tipping or over-tilting in a rearwarddirection relative to the power add-on device.

With reference to FIG. 12, some embodiments of the rotation resistanceelement 602 can have a bracket assembly 606 and roller member 610secured to a distal end portion 606 a of the bracket assembly. As willbe discussed, the roller member 610 can contact a rear portion of thewheelchair and provide at least one of the contact points between therotation resistance element 602 and the wheelchair. The roller member610 can rotate in response to the contact between the wheelchair and therotation resistance element 602, as the wheelchair rotates relative tothe rotation resistance element and/or the rotation resistance elementrotates relative to the wheelchair. In some embodiments, the bracketassembly 606 can have a first bracket element 612 and a second bracketelement 614. The first bracket element 612 can support the roller member610. As will be discussed, the first and second bracket elements 612,614 can be fastened to one another and/or to the power add-on deviceusing any suitable fastening mechanisms, including without limitationthreaded fasteners such as bolts and nuts. For example, a fastener canbe used to fasten the first bracket member 612 to the second bracketmember 614 through opening 616. One example of a fastener that can beused to attach the first bracket member 612 to the second bracket member614 through opening 616 is the threaded fastener 618 shown in FIG. 18. Asimilar threaded fastener 618 can be used to fasten the first end 620 aof the spring member 620 to the first bracket member 612, as illustratedin FIG. 18. Similarly, similar fasteners such as threaded fasteners 619can be used to secure the bracket assembly 606 to the power add-ondevice, as shown in FIG. 17.

With reference to FIG. 13, some embodiments of the rotation resistanceelement can have a spring mechanism 620, such as but not limited to agas spring component, secured to the bracket assembly 606. For exampleand without limitation, a first end portion of the spring mechanism 620a can be secured to the first bracket member 612, and a second endportion of the spring mechanism 620 b can be secured to the secondbracket mechanism 614. Openings 622 can be used to secure the bracketassembly 606 to a frame portion or other support member on the poweradd-on device.

Any embodiments of the power add-on device disclosed herein can have oneor more rotation resistance elements 602 to resist the relative rotationof the power add-on device relative to the wheelchair. In someembodiments, as in the embodiments illustrated in FIGS. 10, 11, and 16,inter alia, the power add-on device can have two rotation resistanceelements 602, one on each lateral side of the power add-on device.Additionally, though not required, the first rotation resistance element602 a and the second rotation resistance element 602 b can beinterconnected with a lateral support member 630 that can be, but is notrequired to be, secured to the bracket assembly 606 of each of the firstrotation resistance element 602 a and the second rotation resistanceelement 602 b.

As shown in FIG. 19, some embodiments of the power add-on device can beconfigured such that the rotation resistance element 602 can be alignedwith a frame portion of the wheelchair, designated with “W” in FIG. 19.For example, the rotation resistance element 602 can be positioned suchthat a centerline 634 of the roller member 610 can be approximatelyaligned with a centerline 634 of a vertical tube member of thewheelchair. In some embodiments, the roller member 610 can be alignedwith any suitable member or surface on the wheelchair, whether it be theframe or otherwise.

FIG. 20 shows a side view of a portion of the rotation resistanceelement 602 in a first or relaxed position relative to a wheelchair.FIG. 21 shows a side view of a portion of the rotation resistanceelement 602 in a second or rotated position relative to a wheelchair. Ascan be seen with reference to FIGS. 20 and 21, rotating the wheelchair610 in the direction designated by arrow A1 relative to the power add-ondevice can cause the first bracket 612 of the rotation resistanceelement 602 to rotate in the direction A1 about the fastener or axlepositioned in opening 616 relative to the second bracket 614. As aresult, the second bracket 612 can compress the spring member 620, whichcan be configured to resist the rotation of the first bracket member 612about the fastener or axle positioned in opening 616. In thisarrangement, the rotation resistance element 602 can resist and slow therotation of the wheelchair relative to the power add-on device 600. Anyof the features of the power add-on device 600 and/or the rotationresistance element 602 can be used with any of the other features orcomponents of any of the other power add-on device embodiments disclosedherein.

While this invention has been described in conjunction with the variousexemplary embodiments outlined above, it is evident that manyalternatives, modifications and variations will be apparent to thoseskilled in the art. Accordingly, the exemplary embodiments of theinvention, as set forth above, are intended to be illustrative, notlimiting. Various changes may be made without departing from the spiritand scope of the invention.

What is claimed is:
 1. A power add-on device for powering a manualwheelchair having rear wheels, comprising: a support structure; a set ofdrive wheels supported by the support structure; a motorized componentsupported by the support structure, the motorized component comprisingdual electric motors and a power source electrically coupled to theelectric motors, wherein each of the electric motors is configured toturn a respective one of a set of drive wheels to propel the manualwheelchair removably coupled with the power add-on device; an engagementmechanism adapted to selectively and removably couple the power add-ondevice to a portion of the manual wheelchair, the engagement mechanismbeing configured to engage with the wheelchair in a manner that permitsthe wheelchair to rotate relative to the engagement mechanism; and arotation resistance element comprising a damping member configured toinhibit rotation of the wheelchair relative to the power add-on deviceand/or rotation of the power add-on device relative to the wheelchair;wherein: the rotation resistance element is movable between a firstposition and a second position; the rotation resistance element isconfigured to move in a first direction toward the second position inresponse to a force exerted on the rotation resistance element from thewheelchair; the damping member is configured to bias the rotationresistance element in a second direction toward the first position andto dampen the movement of the rotation resistance element in the firstdirection, the second direction being opposite to the first direction.2. The power add-on device of claim 1, comprising a second rotationresistance element supported by the support structure, wherein therotation resistance element and the second rotation resistance elementare configured to contact the wheelchair at different locations.
 3. Thepower add-on device of claim 2, wherein the rotation resistance elementand the second rotation resistance element are interconnected by alateral support member.
 4. The power add-on device of claim 1, whereinthe rotation resistance element is configured to rotate about a shaftsecured to the support structure.
 5. The power add-on device of claim 1,wherein the rotation resistance element further comprises a rollermember configured to contact a rear surface of the wheelchair engagedwith the power add-on device.
 6. The power add-on device of claim 1,further including an anti-tip mechanism, the anti-tip mechanismcomprising at least one of a front anti-tip castor wheel coupled withthe support structure and a back anti-tip castor wheel coupled with thesupport structure.
 7. The power add-on device of claim 1, furtherincluding a set of armrests supported by the support structure.
 8. Thepower add-on device of claim 1, wherein the engagement mechanism isconfigured to surround a camber tube of the manual wheelchair, wherein:the wheelchair has one or more front wheels; and the engagementmechanism is configured to permit the camber tube to rotate when the oneor more front wheels of the manual wheelchair are lifted.
 9. The poweradd-on device of claim 1, wherein, when attached to the manualwheelchair, the drive wheels and the rear wheels of the manualwheelchair are substantially aligned in a forward/aft position, suchthat an axle or axles of the drive wheels are substantially aligned in aforward/aft direction relative to an axle or axles of the rear wheels ofthe manual wheelchair.
 10. The power add-on device of claim 1, whereinthe engagement mechanism is further adapted to couple the power add-ondevice to a camber tube of the manual wheelchair.
 11. The power add-ondevice of claim 10, further comprising a clamshell engagement mechanism,wherein, when the clamshell engagement mechanism is in a closedposition, the camber tube of the manual wheelchair is surrounded by theclamshell engagement mechanism.
 12. The power add-on device of claim 1,wherein the engagement mechanism includes at least one electricactuator.
 13. The power add-on device of claim 1, wherein the engagementmechanism includes at least one electric actuator controllable by acontroller.
 14. The power add-on device of claim 1, wherein theengagement mechanism comprises a clamshell engagement mechanism.
 15. Thepower add-on device of claim 14, wherein the clamshell engagementmechanism includes an upper clamshell portion and a bottom clamshellportion, the upper clamshell portion being hingeably attached to thebottom clamshell portion.
 16. A power add-on device for powering amanual wheelchair having rear wheels, comprising: a support structure; aset of drive wheels supported by the support structure; a motorizedcomponent supported by the support structure, the motorized componentcomprising at least one electric motor and a power source electricallycoupled to the electric motors, wherein each of the electric motors isconfigured to turn a respective one of a set of drive wheels to propelthe manual wheelchair removably coupled with the power add-on device; anengagement mechanism adapted to selectively and removably couple thepower add-on device to a portion of the manual wheelchair, theengagement mechanism being configured to engage with the wheelchair in amanner that permits the wheelchair to rotate relative to the engagementmechanism; and a rotation resistance element configured to inhibitrotation of the wheelchair relative to the power add-on device and/orrotation of the power add-on device relative to the wheelchair; whereinthe rotation resistance element further comprises a spring memberconfigured to resist the rotation of the rotation resistance elementrelative to the support structure.
 17. The power add-on device of claim16, wherein the engagement mechanism is configured to surround a cambertube of the manual wheelchair, wherein: the wheelchair has one or morefront wheels; and the engagement mechanism is configured to permit thecamber tube to rotate when the one or more front wheels of the manualwheelchair are lifted.
 18. The power add-on device of claim 16, whereinthe engagement mechanism is further adapted to detach the power add-ondevice from a camber tube of the manual wheelchair.
 19. The power add-ondevice of claim 16, the at least one electric motor comprises dualelectric motors, wherein the power source is electrically coupled to thedual electric motors and each of the dual electric motors is configuredto turn a respective one of a set of drive wheels to propel the manualwheelchair removably coupled with the power add-on device.
 20. The poweradd-on device of claim 16, comprising a second rotation resistanceelement supported by the support structure, wherein the rotationresistance element and the second rotation resistance element areconfigured to contact the wheelchair at different locations and whereinthe rotation resistance element and the second rotation resistanceelement are interconnected by a lateral support member.
 21. The poweradd-on device of claim 16, wherein the rotation resistance element isconfigured to rotate about a shaft secured to the support structure. 22.The power add-on device of claim 16, further including a set of armrestssupported by the support structure.
 23. The power add-on device of claim16, wherein the engagement mechanism includes at least one electricactuator controllable by a controller and the engagement mechanism isadapted to couple the power add-on device to a camber tube of the manualwheelchair.
 24. The power add-on device of claim 16, wherein theengagement mechanism comprises a clamshell engagement mechanism,wherein, when the clamshell engagement mechanism is in a closedposition, the camber tube of the manual wheelchair is surrounded by theclamshell engagement mechanism.
 25. A power add-on device for powering amanual wheelchair having rear wheels, comprising: a support structure; aset of drive wheels supported by the support structure; a motorizedcomponent supported by the support structure, the motorized componentcomprising at least one electric motor and a power source electricallycoupled to the one or more electric motors, wherein each of the one ormore electric motors is configured to turn a respective one of a set ofdrive wheels to propel the manual wheelchair removably coupled with thepower add-on device; an engagement mechanism adapted to selectively andremovably couple the power add-on device to a portion of the manualwheelchair, the engagement mechanism being configured to engage with thewheelchair in a manner that permits the wheelchair to rotate relative tothe engagement mechanism; and a rotation resistance element configuredto inhibit rotation of the wheelchair relative to the power add-ondevice and/or rotation of the power add-on device relative to thewheelchair; wherein the rotation resistance element further comprises aroller member configured to contact a rear surface of the wheelchairengaged with the power add-on device.
 26. The power add-on device ofclaim 25, wherein the engagement mechanism is configured to surround acamber tube of the manual wheelchair, wherein: the wheelchair has one ormore front wheels; and the engagement mechanism is configured to permitthe camber tube to rotate when the one or more front wheels of themanual wheelchair are lifted.
 27. The power add-on device of claim 25,the at least one electric motor comprises dual electric motors, whereinthe power source is electrically coupled to the dual electric motors andeach of the dual electric motors is configured to turn a respective oneof a set of drive wheels to propel the manual wheelchair removablycoupled with the power add-on device.
 28. The power add-on device ofclaim 25, comprising a second rotation resistance element supported bythe support structure, wherein the rotation resistance element and thesecond rotation resistance element are configured to contact thewheelchair at different locations and wherein the rotation resistanceelement and the second rotation resistance element are interconnected bya lateral support member.
 29. The power add-on device of claim 25,wherein the rotation resistance element is configured to rotate about ashaft secured to the support structure.
 30. The power add-on device ofclaim 25, further including a set of armrests supported by the supportstructure.
 31. The power add-on device of claim 25, wherein theengagement mechanism includes at least one electric actuatorcontrollable by a controller and the engagement mechanism is adapted tocouple the power add-on device to a camber tube of the manualwheelchair.
 32. The power add-on device of claim 25, wherein theengagement mechanism comprises a clamshell engagement mechanism,wherein, when the clamshell engagement mechanism is in a closedposition, the camber tube of the manual wheelchair is surrounded by theclamshell engagement mechanism.
 33. The power add-on device of claim 25,further including an anti-tip mechanism, the anti-tip mechanismcomprising at least one of a front anti-tip castor wheel coupled withthe support structure and a back anti-tip castor wheel coupled with thesupport structure.